mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Horizontal Gene Transfer of Plant Parasitism Genes in Nematodes
Animals, for the most part, lack the innate ability to completely break down plant tissue. They lack the enzymes necessarty for cellulose catabolism , and this poses a potential problem when it comes to utilizing plant material as a main food source. Most animals that are able to utilize plants as a sole food source (e.g., cows) are able to do so because they have evolved symbiotic relationships with other organims that have the ability to break down the components of plant tissue that they are unable to digest. Cows and other ruminants have a specialized gut flora containing anaerobic bacteria that are able to break down cellulose. It has long been assumed that animals have not evolved the ability to break down plant cell walls on their own. It is for this reason that the evolution of the ability to metabolize cell walls within the phylum Nematoda is particularly fascinating http://apsjournals.apsnet.org/doi/abs/10.1094/MPMI-03-11-0055. Throughout the evolutionary history of the nematodes, the ability to metabolize plant cell wall components and thereby parasitize plants has originated independently at least three times. The first plant parasitic nematode (PPN) genes that were characterized were found to contain functional, endogenous cellulase enzymes. These enzymes were found in the PPNs subventral esophageal gland cells, and it was shown that the PPNs used these enzymes to break down plant cell walls http://www.ncbi.nlm.nih.gov/pmc/articles/PMC20186/. When the nematode cellulase protein genes were first discovered, it was shown that they had a striking similarity to bacterial genes. Furthermore, no other nematodes that had been studied up until that point exhibited these cellulase genes. These findings led to the hypothesis that the PPN cellulase gens had be acquired via horizontal gene transfer (HGT) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC33851/. Horizontal Gene Transfer (HGT) in Nematodes Has Led to the Ability to Parasitize Plants Horizontal gene transfer (HGT) is defined as "the asexual movement of genetic material between different species, regardless of the evolutionary distance between them" 1. The phenomenon of HGT is quite common in bacteria. In fact, it has been shown that genomic DNA as a consequence of HGT comprises a relatively large percentage of most bacterial genomes. Conversely, thus far it is uncommon to see cases of reported HGT between animals and microbes possibly due to the fact that in order for a microbe to animal HGT to be successful it must occur in the germline, and exposure of germline tissues to exogenous DNA is an uncommon event. Furthermore, a successful HGT event requires that the exogenous DNA to be incorporated into the host species genome must be able to produce at least one functional protein product, and this protein would need to confer some sort of evolutionary advantage in order for it to remain fixed in the population 2, 3. HGT Fig 1.jpg|Fig. 1. Schematic representation of the phylogeny of Nematoda. HGT Fig 2.jpg|Fig. 2. Distribution of genes acquired via horizontal gene transfer (HGT) in the phylogeny of plant-parasitic nematodes (PPN) from clades 12 and 10b. HGT Fig 3.jpg|Fig. 3. In situ hybridization image showing expression of a Globodera pallida cellulase. Genome sequencing or large-scale expressed sequence tag (EST) projects have identified multiple candidate genes acquired by HGT, and these studies have been conducted on many species of PPN. The parameters necessary to confirm HGT include phylogenetic methods and the overlap of habitats between donor and recipient organisms 3. The clade of PPN that has the most economical significance is clade 12, which includes the cyst nematodes (CN) and the root-knot nematodes (RKN) (Figure 1). Clade 12 has been thoroughly studied by both genomics and transcriptomics approaches, and as a result Consequently, there is much more information about clade 12 in the published literature than exists for other clades (Figs. 1 and 2, clades 10b, 1, and 2) 1. Thorough analyses of clade 12 have indicated that a high rate of HGT has occurred throughout the course of the evolution of plant parasitism. Multiple plant cell wall–modifying proteins have been identified in these PPN, including cellulases, pectate lyases, xylanases, polygalacturonases, arabinogalactan galactosidases, arabinanases, and expansin-like proteins (Figure 2) 1. All of the proteins that have been identified as having arisen in PPN as a result of HGT are expressed in the subventral gland cells of the PPN (Figure 3). These proteins and enzymes function to enable the PPN to degrade plant cell walls, and the PPN use this together with a specialized structure they have called a stylet which allows them to mechanically puncture the plant cell walls. Once a cell wall has become softened or compromised the PPN are able to invade and migrate in the intercellular environment where they feed off of the plant 1. The occurrence of HGT events in PPN is an exciting biological phenomenon. Particularly interesting is that these are members of the Animal kingdom that have acquired the ability to break down cellulose themselves without the aid of a symbiont. It is well established that HGT events have played important roles in the evolution of the prokaryotes, but know scientists are beginning to accept, and further investigate, the idea that HGT events may also have a significant role in the evolution of eukaryotes. References 1. Haegeman A, et al. (2011) Horizontal Gene Transfer in Nematodes: A Catalyst for Plant Parasitism? Molecular Plant-Microbe Interactions ''(24)8:879-887. 2. Smant G, ''et al. (1998). Endogenous cellulases in animals: Isolation of β-1,4-endoglucanase genes from two species of plant-parasitic cyst nematodes. Proc. Natl. Acad. Sci. 95:4906-4911. 3. Keen, NT & Roberts, PA. (1998). Plant parasitic nematodes: Digesting a page from the microbe book. Proc. Natl. Acad. Sci. 95:4789-4790.